1. Technical Field
The present invention relates to novel beam welding apparatus and a technique to perform reliable welding as well as improving the appearance of a welded portion.
2. Related Art
Beam welding, for example laser welding, has attracted people's attention because it offers various advantages concerning the joining of a housing and a transparent cover of a vehicular lighting device over the joining techniques including joining, hot plate welding, and welding by way of oscillation of ultrasonic waves or the like that have been practiced in the related art.
For example, in the case where a housing and a transparent cover of a vehicular lighting device are joined together via laser welding, the exemplary process mentioned below is used because the housing and the transparent cover are relatively large and the joining site has a complicated three-dimensional shape. A laser gun is supported by a 6-shaft articulated robot to perform control of the position and posture of the laser gun. A housing and a transparent cover overlaid one on the other are rotated in a horizontal plane (with vertical movement as appropriate) as the laser gun irradiates laser beams while scanning a welding line (this continues through all of the positions where the welding part of the housing and the welding part of the transparent cover are in contact with each other).
As mentioned above, the housing and the transparent cover are relatively large and the joining site has a complicated three-dimensional shape. Besides that, variations in the dimensions or shape of each product, the problems of teaching a scanning robot, swinging of a controlled robot and other problems may cause laser irradiation off the welding line.
For example, as shown in FIG. 3A, laser beams fare successfully irradiated by a laser gun a onto the welding surface e of a housing d in contact with the welding surface of a transparent cover b. As shown in FIG. 3B, when the laser gun is deflected inward, the laser beams f are irradiated onto a portion g inside the welding surface e of the housing d thus causing a scorch on the portion g. As shown in FIG. 3C, when the laser gun is deflected outward, the laser beams f are irradiated onto a portion h outside the welding surface e of the housing d thus causing a scorch on the portion h. In FIG. 3, a sign i represents a cover fixing jig for pressing the transparent cover b toward the housing d in order to secure mutual contact between the welding surfaces c and e.
A scorched portion g or h mentioned above results in a considerably poor appearance. Moreover, a gas may be generated when a scorch occurs. The gas may cause a fog on the transparent cover b. This presents a problem with safety.
In case the laser gun a is deflected farther inward or outward than shown in FIG. 3B or 3C, the irradiation position of the laser beams is completely dislocated from the welding line and the target position is left non-welded. Such a welding-lacking portion prevents a space defined by the housing and the transparent cover from being enclosed thus impairing the water-resistance.